1. Field of the Invention
This invention relates generally to the field of noise suppression and, more particularly, to the suppression of jet noise by means of a hot gas shield. Although this invention finds particularly useful application in the suppression of noise from a supersonic aircraft, it is expressly to be understood that the advantages of the invention are equally well manifest in other types of jet aircraft and other fields wherever and whenever similar conditions and problems exist, for example, in subsonic or transonic jet engine applications.
2. Background of the Invention
Increasing public awareness of and attention to environmental quality has generated an ongoing regulatory process intended to decrease the level of permissible noise in the environment. One area of concern in this process is the intense noise that may be generated by jet aircraft when they are close to the ground, particularly on take-off and climb. While this problem has plagued both the aircraft industry and the public since the introduction of jet aircraft, increased attention recently has been focused on the problem by the introduction of supersonic transport aircraft (SST). The problem is rapidly reaching a point at which noise regulations could seriously delay, if not entirely preclude, technological advancement in the field. The instant invention offers a technically acceptable, environmentally sound solution to this problem.
Many different approaches to the problem have been suggested including acoustic linings of various types, internal mixers, multi-tube nozzles, ejector rings and noise-optimized engine performance parameters. Although many of these proposals have been effective to reduce or suppress noise to some degree, they generally have not achieved the desired reductions and have frequently produced significant weight and/or performance penalties.
Another proposal, more closely related to the instant invention, is the suggestion of a hard, or mechanical shield, extending a distance downstream of the engine exit sufficient to significantly affect the noise level perceived by a ground observer. This approach proved to be generally impractical because of added weight as well as drag penalties, if the shield remained deployed during cruise when noise suppression was not necessary. Attempts to provide a hard shield that would retract at cruise failed due to the complexity and added weight of the retracting mechanism. Also, the hard shield could not totally be made acoustically reflective, allowing some noise to reach the observer.
Although the refraction and reflection effects of the fluid media through which sound waves travel have long been recognized, as evidenced by Lord Raliegh's Theory of Sound published in 1878, few practical applications of these effects have been proposed. However, it has recently been suggested that a fluid shield could have noise suppression properties similar to those of a hard shield without the associated drag penalty. While the sound suppression potential of a fluid shield generally has been confirmed, attempts to create a practical generator which produces a shield having the desired sound attenuation properties have met with little success. Because optimum shield temperatures are believed to be relatively high, it has been suggested that additional burners could be used to generate a shield layer. Once again unacceptable weight penalties result from the fuel required by the burners as well as the burner structures themselves. Another proposal suggests ducting the hot turbine air outside the cooler fan air to produce a shielding effect. This so-called "inverted flow" engine does, to some extent, reduce noise, but at a significant loss in thrust from the flow inversion process. Additionally, other shield properties such as shield geometry, pressure ratio and velocity are not necessarily optimized in the inverted flow engine. Another approach suggests producing a shield having sound suppression and/or absorption properties due to its lower molecular weight which is caused by introducing a material, e.g., water vapor, into the air stream. This obviously requires added weight to be carried in the form of a water supply, conduit and injector mechanisms. A related concept suggests the injection of a gas such as helium to create a fluid layer having noise suppression properties. This approach also suffers from the disadvantages of added weight due to generator supplies, such as storage tanks, conduit, etc., as well as the necessity of estimating and transporting sufficient quantities of the gas for an entire mission.
It is, accordingly, an object of the present invention to provide a method and apparatus for generating a noise attenuating fluid shield that is not subject to these and other disadvantages and limitations of the prior art.
It is another object of the invention to provide a method and apparatus for generating a noise suppressing fluid shield that ulitizes engine exhaust air appropriately conditioned or modified to produce a shield having near-optimum noise suppression parameters.
It is a further object of the invention to provide a method and apparatus for generating a fluid shield that is lightweight, simple in operation, requires few moving parts and therefore is easily adapted to a variety of jet engines.
It is yet another object of the invention to provide a fluid shield generating method and apparatus that redirects engine exhaust air and reduces the pressure thereof to produce an appropriately located shielding layer.